The present invention relates to a structure and process for injection molding optical and compact disks.
Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal materials such as iron, steel, stainless steel, aluminum, or brass because these materials have high thermal conductivity and thus permit the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. A drawback to rapid cooling in these molds is that the injected resin freezes instantaneously at the mold surface, resulting in a thin solid layer. Quick quenching of the melt at the mold surface creates a rough surface (instead of a high quality optical surface) which can impact disc performance. The quick solidification of the melt combined with variable radial flowability of the materials makes the uniform melt flow and uniform surface replication required for an optical disk difficult to achieve. Non-uniform flow and surface imperfections can result in areas on an optical disk with high bit errors.
In the injection molding of compact discs, for audio, video, or computer data storage and retrieval applications, heat transfer through the mold thus has a strong effect on molding time and disc attributes such as birefringence, flatness, and accuracy of feature replication. For a process to be economical, a careful balance must be maintained between low cycle times and the process parameters needed to meet the quality requirements.
A method for affecting heat transfer and improving the cycle time during injection molding by incorporating insulation into the mold has been described in commonly assigned Kim et al., U.S. Pat. No. 5,458,818. In Kim et al., a multilayer mold is used in which a metal core has an insulating layer bonded thereto for slowing the initial cooling of the resin during the molding operation. The insulating layer comprises material having both low thermal diffusivity and conductivity, thus slowing the cooling of the molded resin, and good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. One or more skin layers of hard material, typically metal, can be bonded to the insulating layer.
Another method for affecting heat transfer is described in Nakamura et al., Japanese Unexamined Patent Application Disclosure Bulletin No. 88-71325. In Nakamura et al., a layer of synthetic resin is formed on a stamper by coating or lamination before the stamper is placed on the core molding surface of the metal mold.
The use of an insulating layer is desirable so as to cause a minimal change in the size and shape of the molding tool and equipment. For compact discs, stringent requirements of optical clarity, surface morphology, and replication of surface features of sub-micron dimensions obviate the use of common insulating materials, which do not provide a smooth enough surface, are not stable for long periods at the mold temperature, or cannot withstand the repeated application of high pressure during the molding process.
For a sheet or film to be useful for managing heat transfer for a mold it must have a very smooth surface ( less than 0.1 xcexcm surface roughness) over a large area so that it will not introduce feature replication errors or surface imperfections into the manufactured disk. It is also preferred that the surface be compliant to attenuate minor imperfections in the molding tool while maintaining mechanical and dimensional integrity during the molding process.
It is therefore seen to be desirable to provide a structure and method for molding optical disks having improved surface replication and improved molding characteristics.
Briefly, in accordance with one embodiment of the present invention, a method for molding an optical disk comprises: applying a thermally insulative mold insert onto a thermally conductive mold form by coating the mold insert on the mold form, the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form and comprising an adhesion promoting material; positioning the coated mold form in a thermally conductive mold apparatus with the mold insert positioned between the mold form and the mold apparatus; injecting a molten thermoplastic material into the mold apparatus; retaining the molten thermoplastic material in the mold apparatus for a time sufficient for the molten thermoplastic material to cool below its glass transition temperature to form the optical disk; and ejecting the optical disk from the mold apparatus.
According to another embodiment of the present invention, a mold insert for being coated on a mold form and positioned in a mold apparatus between the mold apparatus and the mold form has a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form and comprises an adhesion promoting material.
In a related embodiment, an optical disk mold apparatus comprises: at least one thermally conductive mold form at least one thermally insulative mold insert coated onto the thermally conductive mold form, the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form and comprising an adhesion promoting material; and a thermally conductive mold apparatus, with the at least one coated mold insert positioned between the thermally conductive mold form and a portion of the thermally conductive mold apparatus.